Update logging jars

This PR updates the following logging jars slf4j (1.7.6) and logback (1.1.1) and further depends on those jars being in the Bio-Formats repository. Hence it,

--depends-on openmicroscopy/bioformats#966

A `FINALIZE_SESSION_MARKER` has been added to the import log. There should be no changes to existing logging other than a final line in any new import log with the text:
```
Finalizing log file.
```